In ophthalmology, surgical microscopes tailored to the requirements of ophthalmology are frequently used for diagnostic purposes and during surgical interventions. Such surgical microscopes (operating microscopes) are also referred to as ophthalmology surgical microscopes.
Ophthalmology surgical microscopes are optical reflected-light microscopes used during surgical interventions and providing a magnification of typically between 5× and 30×. Surgical microscopes have, as compared to other optical reflected-light microscopes, an objective (objective lens system) with a larger focal length of typically between 175 mm and 550 mm. Characteristic for ophthalmology surgical microscopes is hereby that they often do not directly image an eye's ocular fundus, but indirectly by imaging an intermediate image of the eye which is produced by the eye lens and an ophthalmoscope lens located in the vicinity of the eye. The ophthalmoscope lens is hereby positioned at a distance of 4 mm to 80 mm to the eye's lens. The ophthalmoscope lens may be attached to the ophthalmology surgical microscope in at least one of a pivotable, removable and exchangeable manner. In surgical microscopes, the field of view, i.e. the area in the focal plane from which at least one optical imaging path of the microscope can produce an image at a certain point in time onto the retina of a user of the microscope, is typically larger than 1 mm2. The field of view of a surgical microscope therefore comprises not only one single image point as is the case with scanning microscopes; rather, a multidimensional (especially two or three dimensional) image of the object observed with the surgical microscope is produced at each point in time.
To provide a user with a three dimensional impression of the object to be imaged, which is particularly important with eye surgery, ophthalmology surgical microscopes are often configured as stereo(scopic) microscopes simultaneously providing a pair of optical imaging paths to the eyes of a user. The optical imaging paths of the pair of optical imaging paths intersect near the focal plane of the surgical microscope at a stereoscopic angle of between 3° and 14°.
In ophthalmology surgical microscopes, the image of the object produced with the surgical microscope is provided to a user either by means of an eyepiece (or a pair of eyepieces in stereoscopic surgical microscopes) or is converted into electrical signals using an image converter (or a stereo image converter or a pair of image converters in stereoscopic surgical microscopes), and presented to a user, in addition or alternatively to the eyepieces, on at least one of a monitor and a head-mounted display.
In order to satisfy the ophthalmological requirements, ophthalmology surgical microscopes often comprise an integrated slit lamp and/or a keratoscope.
In ophthalmology it is occasionally necessary to render visible by dyes areas of a patient's eye to be observed. It is for example known to stain the epiretinal (limiting) membrane between the retina and the vitreous body, which itself is colorless and transparent, with indocyanine green, brilliant blue or trypan blue, since these dyes stain the limiting membrane more easily than the adjacent retina and the adjacent vitreous body. The problem is, however, that the respective biocompatibility of the dyes used and in particular that of indocyanine green often allows for only a low dose.
Some of the dyes used in ophthalmology are so called fluorescence dyes which, apart from the general characteristic of dyes of either absorbing or reflecting light of different wavelengths, absorb light of a first wavelength range (the so called excitation band) and convert it into light of a second wavelength range (the so called fluorescence band) different to the first wavelength range. Indocyanine green, for example, absorbs light of a wavelength range from 600 nm to 900 nm, and emits fluorescence in a wavelength range of 750 nm to 950 nm.